The invention applies in particular to monochromators with holographically made concave gratings, that is to say that although the holographic process is the only one at present available for obtaining such gratings, the scope of the present invention is not restricted to this particular process of manufacture.
The principle of the process of manufacturing a grating by holography is already known. Reference may be had, for example, to French Pat. No. 2,036,613.
For the purpose of the present description, it will
.omega. to recall the broad outlines of the manufacturing process with reference to FIG. 1 of the attached drawing. PA1 A (.alpha., l.sub.A) PA1 B (.beta., l.sub.B) PA1 C (.gamma., l.sub.C) PA1 D (.delta., l.sub.D) PA1 .phi. = (.alpha.-.beta.)/2 (where 2.phi. is the angle at which the two slits at the summit of the grating are viewed.)
The process consists of causing two coherent light waves .SIGMA..sub.C and .SIGMA..sub.D emitted from two points C and D and originating from a laser to fall on a layer S under such conditions that the surfaces of interference of the two waves intersect the layer, the light energy concentrated on these surfaces producing sufficient modification of the layer at the zones of intersection.
This layer may be, for example, a layer of photopolymerisable resin deposited on an optically bright, polished surface of a support made of any suitable material and having any suitable form, for example a support made of glass or silica, the sensitive surface of which is concave and may have, for example, the form of a spherical calotte.
A solvent is then applied to dissolve selectively either the resin which has been polymerised by the light energy or the resin which has not been polymerised, so that the lines of the grating appear.
If it is desired to obtain a grating by reflection, the surface of the grating is subsequently metallised under vacuum.
The production of monochromators with such gratings has already been envisaged, see for example French Patent 70 27186.
A disadvantage commonly found in conventional monochromators having a concave grating is that they require a multiplicity of mechanical movements including translations and rotations to obtain the desired separation of the spectrum. A type of monochromator which does not have this disadvantage is known but in it the freedom from this defect is achieved only at the expense of the qualities of resolution and luminosity.